The long range goal of this project is to define physiological and biochemical properties of Acanthamoeba and Naegleria that contribute to virulence of the organisms and to screen drugs that may be effective in chemotherapy of human primary amebic encephalitis. The primary goals of the present project are to characterize molecular differences that may be used to distinguish avirulent and virulent Acanthamoeba, to investigate the mechanism of encystment induction, to determine the effectiveness of 5-fluorocytosine (5-FC) as a chemotherapeutic in experimental mice amebic encephalitis, and to elucidate the biochemical mechanism of the action of 5-FC as an amoebostatic or amoebocidal agent.